- (void)viewDidLoad
{
[super viewDidLoad];

// 1.创建pickerview  
// pickerview有默认的frame  
UIPickerView \*pickerView = \[\[UIPickerView alloc\] init\];  
pickerView.dataSource = self;  
pickerView.delegate = self;  
\[self.view addSubview:pickerView\];  
self.pickerView = pickerView;  

}

#pragma mark - UIPickerViewDataSource
// 告诉系统有多少列

• (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView
  {
  return ;
  }
  // 告诉系统有多少行

• (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component
  {
  if ( == component) {
  // 省份列
  return self.provinces.count;
  }else
  {
  // 城市列
  // 获取第0列选中的行
  NSInteger selectIndex = [self.pickerView selectedRowInComponent:];
  // 1.根据第0列选中的行数获取对应的省
  NJProvince *province = self.provinces[selectIndex];
  // 2.获取对应省份对应的城市
  NSArray *cities = province.cities;
  // 3.返回城市的个数r
  return cities.count;
  }
  }
  #pragma mark - UIPickerViewDelegate
  // 告诉系统每一行显示什么内容

• (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component
  {
  NSLog(@"titleForRow");

  if ( == component) {
  // 省份
  // 1.获取对应行对应的省份模型
  NJProvince *province = self.provinces[row];
  // 2.返回省份的名称
  return province.name;
  }else
  {
  // 城市列
  // 0.获取第0列选中的行数
  NSInteger selectIndex = [pickerView selectedRowInComponent:];
  // 1.获取对应的省份
  NJProvince *province = self.provinces[selectIndex];
  // 2.获取对应的城市
  return province.cities[row];
  }
  }

// 监听pickerView的选中

• (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component
  {
  // 判断是否修改了第0列(省份列)
  if ( == component) {
  // 刷新第1列对应的数据
  [pickerView reloadComponent:];
  // 让第1列滚动到第0行
  [pickerView selectRow: inComponent: animated:YES];
  }
  }